The present disclosure concerns ongoing efforts in developing lactide polymers useable in preferred manners. U.S. Pat. No. 5,142,023 issued to Gruber et al. on Aug. 25, 1992, the disclosure of which is hereby incorporated by reference, discloses, generally, a continuous process for the manufacture of lactide polymers from lactic acid. Selected polymers according to U.S. Pat. No. 5,142,023 have physical properties suitable for replacing petrochemical-based polymers for packaging, paper-coating and other applications. Related processes for generating purified lactide and creating polymers therefrom are disclosed in U.S. Pat. Nos. 5,247,058, 5,247,059 and 5,274,073 issued to Gruber et al., the disclosures of which are hereby incorporated by reference.
Generally, commercial exploitation of polymers utilizing processes such as those disclosed in the patents to Gruber et al. can involve conversion of raw material monomers into polymer beads, resins, or other pelletized or powdered products. The polymer in this form would then be sold to end users who would extrude, blow-mold, cast films, blow films, foam, thermoform, injection-mold, fiber-spin or otherwise convert the polymer at elevated temperatures, to form useful articles. The above processes (and related processes) are collectively referred to herein as "melt-processing". Polymers produced by processes such as those disclosed in the patents to Gruber et al., and which are to be sold commercially as beads, resins, powders or other non-finished solid forms, are herein generally referred to collectively as polymer resins. These polymer resins, if biodegradable, can help alleviate the environmental stress due to disposal of items such as packaging materials, coated paper products, films, single use diapers and the like.
It is generally known that lactide polymers or polylactides are unstable. The concept of instability has both negative and positive aspects. A positive aspect is the relatively rapid biodegradation or other degradation that occurs when lactide polymers or articles manufactured from lactide polymers are discarded or composted after completing their useful life. A negative aspect of such instability is the potential for degradation of lactide polymers during processing at elevated temperatures, for example during melt-processing by end-user purchasers of polymer resins. Thus, some of the same properties that make lactide polymers desirable as replacements for relatively non-degradable petrochemical polymers also can create undesirable effects during production of lactide polymer resins and processing of those resins.
Lactide polymers are subject to unwanted degradation during melt processing via a number of pathways. These pathways include hydrolysis and other side reactions, which, for example, result in lactide formation and decreased molecular weight of the polymer. Furthermore, as processing temperatures are increased (especially to above about 230.degree. C.), lactide polymer degradation is substantially and undesirably accelerated. Accordingly, even if a relatively melt-stable lactide polymer can be produced, it would be generally desirable to provide a lactide polymer or resin formulation that can be processed into useful articles at reduced temperatures (i.e., especially and preferably at no more than about 180.degree. C.).
During certain melt processing operations, linear polymers such as linear polylactide exhibit certain undesired flow properties, such as necking. For example, if polylactide is extruded as a film onto a moving substrate, the film of polylactide being directed onto the substrate will tend to neck under the tensional forces caused by the moving substrate. By "necking" in this context it is meant that the width of the film will tend to narrow as the film is pulled or stretched. This leads to problems with control of the process and problems with maintaining consistency in film thickness, etc. Specifically, in comparison to polypropylene or polyethylene, linear polylactides (PLA) typically exhibit substantially more problem necking and less melt strength. Linear polymers, such as PLA, also tend to exhibit hydrodynamic instability or draw resonance at high draw ratios. This draw resonance can cause a periodic variation in a coating width and/or gauge, for example, and can lead to rupture of the polymer web.
Moreover, in a coating application or blown film production the polymer must withstand various forces such as acceleration in going from the die to the substrate in a coating application or the gas pressure that causes stretching in a blown film. The ability to withstand these forces is referred to as "melt-strength". There has been a need for lactide polymer formulations that will have improved melt-strength.